help with rear diff

[maximum]

I have a newer emaxx that hasn't been bashed or run that much. I took it to the track yesterday and ther rear wheels don't spin. When you turn the rear left tire the right tire does not move at all, but the rear cvd spins. Obviously there is something broke in the diff case. before i take it apart does anyone know what the most probable part that broke is? I looked at the spool diff you can buy is this a good replacement alternative? What would be a good and permanent fix so I don't ever have to go back and rebuild this rear diff. Thanks for your help.